1. Field of the Invention
The present invention relates to earth-working equipment, such as an agricultural implement pulled by a tractor; and more particularly to a method for controlling a hydraulic system that operates a hitch that couples the implement to the tractor.
2. Description of the Related Art
A variety of agricultural implements are available to be pulled by a tractor for working earth in a farm field in which crops will be or have been planted. The implement is connected to a standard three-point hitch with right and left drag links on the rear of the tractor and the hitch can be operated to raise and lower the implement. The hitch is raised and lowered hydraulically by piston-cylinder assembly that is operated by a valve that controls the flow of fluid to and from the piston-cylinder assembly.
A conventional tractor has a control panel by which the operator sets desired upper and lower positions for the hitch and a desired velocity at which the hitch should travel up and down. The operator then activates an input device to select raising or lowering the hitch. An electronic control system responds to that switch by operating an electrohydraulic valve to drive the piston-cylinder assembly so that the hitch moves in the designated direction and speed until the hitch reaches the selected position at which time the valve is closed. Specifically the electronic control system applies a given level of electric current to the electrohydraulic valve which opens the valve a corresponding degree thereby providing a related amount of fluid flow through the valve.
Electrohydraulic hitch valves typically have been designed with a mechanical flow compensator on the raise function. The flow compensator provides a constant flow rate (raise rate) at a given valve current regardless of the load on the hitch arms and regardless of other pressure demands of the hydraulic system. Flow compensation usually is not implemented on lower function of the hitch valve. This results in variable lowering rates for a given valve current. The lower rate varies due to different loads being placed on the hitch, as well as due to changes in hitch geometry as the hitch arms change position. The use of a mechanical flow compensation technique similar to that used during raise could provide a constant lowering rate for a given valve current, but doing so would add cost and complexity to the valve assembly.
As a consequence, there is a need for a hydraulic control system that provides flow compensation during both raise and lower operations.